AuthorTopic: ETTR vs ISO (Read 46604 times)

In order to avoid such confusion, I think it is better to consider exposure as something which is determined only by F/stop, shutter speed

The ISO setting is merely an instruction to the camera's internal processing electronics to process an exposure in a particular way.

Well said Ray. 'Processing' in the sense of amplifying the signal rolling off the sensor before writing it to the Raw data (making it 'brighter'). Amplification can be done analogically and/or digitally. If done digitally, there is no difference whether the signal is increased in-camera (through a higher ISO) or later in the Raw converter (through a +'EV' correction) - so might as well keep the lower ISO and the extra Dynamic Range and correct brightenss later in the raw converter. For instance the D800e applies only digital amplification from about 1600 ISO on up as can be seen in these Raw histograms:

The ISO 800 Raw histogram of the green channel is fully populated, the ISO 1600 is showing a small amount of digital amplification, the 3200 and 6400 are simply the ISO 1600ish ADC integer output multiplied digitally by 2 and 4 - with every second and every fourth value present respectively. There is no difference between doing the multiplication in-camera or later in the raw converter through +EC correction, except that in-camera one may clip desirable highlights which one may regret later. The Raw converter needs to be well behaved for extreme (>3 stops?) recoveries to work as expected (RT apparently is while LR apparently is not).

That's digital amplification. What about the analog component, that is when increasing ISO will amplify analogically the signal rolling off the sensor before feeding it to the ADC (think of it as turning up the volume on your stereo)? Some cameras' shadow SNR, in particular those whose ADC's noise floor is higher than the sensor's (e.g. Canons), will benefit from boosting the signal from the sensor before it is fed to the ADC, so in their case it may be worth compromising DR in exchange for better shadow SNR.

This suggests the following strategy, as mentioned by emil and others: First max out Exposure at base ISO by choosing the longest shutter speed and widest aperture allowed by your technical/artistic constraints, thereby gathering the largest number of photons possible for a given blur, DOF and highlight retention objective. Then you may choose to raise ISO as a compromise between minimizing shadow SNR and maximizing DR. Bill Claff has worked out where increasing ISO no longer helps shadow SNR for various cameras. No point increasing ISO beyond 1600 for the D800e (really ISO 400 if one is willing to live with a 1/6 of a stop penalty in shadow SNR for the extra two stops of DR), although if you own a 5DIII you may not want to stop before ISO 2500 or higher - if needed. If the above strategy for the D800e results in a darker image OOC than usual, no sweat: simply correct for pleasing brightness/tones in the raw converter, confident that you have the best IQ possible.

Once one realizes that Exposure is all about getting the most photons possible given one's technical/artistic constraints, and ISO is an independent variable about processing those photons into a raw file while maximizing SNR and DR without clipping desirable highlights one is ready to go.

Well said Ray. 'Processing' in the sense of amplifying the signal rolling off the sensor before writing it to the Raw data (making it 'brighter'). Amplification can be done analogically and/or digitally. If done digitally, there is no difference whether the signal is increased in-camera (through a higher ISO) or later in the Raw converter (through a +'EV' correction) - so might as well keep the lower ISO and the extra Dynamic Range and correct brightenss later in the raw converter. For instance the D800e applies only digital amplification from about 1600 ISO on up as can be seen in these Raw histograms:

Do you know if that graph has been formulated for cameras other than the D800?

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The ISO 800 Raw histogram of the green channel is fully populated, the ISO 1600 is showing a small amount of digital amplification, the 3200 and 6400 are simply the ISO 1600ish ADC integer output multiplied digitally by 2 and 4 - with every second and every fourth value present respectively.

This I understand.

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That's digital amplification. What about the analog component, that is when increasing ISO will amplify analogically the signal rolling off the sensor before feeding it to the ADC (think of it as turning up the volume on your stereo)? Some cameras' shadow SNR, in particular those whose ADC's noise floor is higher than the sensor's (e.g. Canons), will benefit from boosting the signal from the sensor before it is fed to the ADC, so in their case it may be worth compromising DR in exchange for better shadow SNR.

This I also understand.

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This suggests the following strategy, as mentioned by emil and others: First max out Exposure at base ISO by choosing the longest shutter speed and widest aperture allowed by your technical/artistic constraints, thereby gathering the largest number of photons possible for a given blur, DOF and highlight retention objective. Then you may choose to raise ISO as a compromise between minimizing shadow SNR and maximizing DR. Bill Claff has worked out where increasing ISO no longer helps shadow SNR for various cameras. No point increasing ISO beyond 1600 for the D800e (really ISO 400 if one is willing to live with a 1/6 of a stop penalty in shadow SNR for the extra two stops of DR), although if you own a 5DIII you may not want to stop before ISO 2500 or higher - if needed. If the above strategy for the D800e results in a darker image OOC than usual, no sweat: simply correct for pleasing brightness/tones in the raw converter, confident that you have the best IQ possible.

However I'm not sure I understand how to read the graph?

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RFPhotography

Dreed, the article linked at the bottom of the page Jack links titled 'Sensor Analysis Primer - Photographic Dynamic Range Shadow Improvement' has the information on how to read the chart. Basically, where the curve flattens out is the point where increasing ISO no longer has any benefit on shadow dynamic range.

In order to avoid such confusion, I think it is better to consider exposure as something which is determined only by F/stop, shutter speed, and that quality we often ignore, T/stop, which relates to the transmissive qualities of the glass in the lens.

The ISO setting is merely an instruction to the camera's internal processing electronics to process an exposure in a particular way.

+1To avoid arguments that are more about language than photographic technique, let us agree to use words like "exposure" in standard agreed way (like "the amount of light reaching the sensor") and move on the the practical issues.

Unfortunately "ETTR" has acquired two different and widely used meanings, so we should distinguish betweena) maximizing the exposure received by the sensor (stopping just short of blowing highlights by overfilling any photosites).b) maximizing the output levels of the ADC, by some combination of exposure to the sensor and analog amplification of the sensor output before anaog-to-digital conversion.

+1To avoid arguments that are more about language than photographic technique, let us agree to use words like "exposure" in standard agreed way (like "the amount of light reaching the sensor") and move on the the practical issues.Unfortunately "ETTR" has acquired two different and widely used meanings, so we should distinguish betweena) maximizing the exposure received by the sensor (stopping just short of blowing highlights by overfilling any photosites).b) maximizing the output levels of the ADC, by some combination of exposure to the sensor and analog amplification of the sensor output before anaog-to-digital conversion.

Call it optimal exposure. I know there's been one for transparency film which you probably want to nail within ⅓ to ˝ a stop (some would argue that is too much). Using the same (and good) E6 line.

Speaking of the full process to get the image, the labs or E6 lines could differ that amount although not ideal. Kind of like this digital ISO discussion. Amplification or lack of chemistry? I stuck with one very good lab for years (A&I) and tested every bulk purchase of E6 with them, it was part of the entire 'expose film' workflow.

I suggest trying a few with your intended image type to see what works for you. You may not notice or you might like the non-neutral rendition. Most are fine with linear brightness increases (+EC) of a few stops so I wouldn't worry about it. If you find undesirable hue twists or other raw-converter introduced artifacts at high ISOs I understand that RawTherapee with the 'neutral' profile is as 'clean' as it gets, and it lets you correct up to 10 stops.

+1To avoid arguments that are more about language than photographic technique, let us agree to use words like "exposure" in standard agreed way (like "the amount of light reaching the sensor") and move on the the practical issues

Glad you agree with me on this point, BJL, because we've have had a few disagreements in the past.

Another issue which I think is also important is getting a grasp on the practical significance of any differences of exposure in conjunction with differences in ISO settings.

One may understand in terms of a theoretical concept that A is better than B, but in order to determine the practical significance of such differences, one might need to do one's own experiments and comparisons, rather than just accept the theoretical model and behave accordingly.

An example would be the way one uses a Canon 50D. This camera is specified as having an ISO range from 100 to 12,800, with ISOs above 3200 being expanded. What this means is that there is nothing to be gained regards fundamental image quality, by using ISO 12,800. If one uses the same exposure (as defined by f/stop and shutter speed) at ISO 3200, as what one would consider to be the correct, or optimal, or ETTR exposure at ISO 12,800, then the camera's LCD screen and histogram will show an apparent underexposure of 2 stops.The image will look dark, but after appropriate processing in the RAW converter the ISO 3200 shot should have equally good shadow detail as the 12,800 shot. Or to be more precise, equally bad shadow detail.

So what are the practical consequences here, one might ask. As I see it, the ISO 12,800 shot produces a review on the camera's LCD screen which can be seen more clearly. Have you captured what you want, or do you have to take another shot?The ISO 3200 image will likely be a bit hopeless for such assessments. The main advantage will be, there is far less risk of blowing highlights unintentionally if one uses the same exposure at ISO 3200.

Another example of the importance of conducting one's own tests, which is illustrated by the peculiarities of the 50D, is the practical significance of using the ISO 100 setting as opposed to ISO 200 on the 50D. There's no doubt that the same exposure (as defined by F/stop and shutter speed) at ISO 200 will provide lower shadow noise than you would get at ISO 100. But what happens if we compare an ETTR exposure at ISO 100 with an 'apparent' ETTR exposure at ISO 200? In these circumstances the sensor receives twice the amount of light at ISO 100. Shadow detail must surely be better.

For a whole year or more, after buying a Canon 50D, taking photos in places like museums, churches and art galleries where flash and tripods were not allowed, I sometimes struggled to hold the camera steady, often preferring to use ISO 100 with the widest aperture on the lens instead of the sharpest aperture on the lens, thus compromising resolution at least a bit, because I imagined I'd be getting better shadow detail and better SNR at ISO 100 with the 50D.

Some time later, after DXO had made its test results freely available to the public, and I was in the market for an upgraded camera, I happened to compare the performance of the Canon 50D with other more recent models, using the DXOMark graphs for ISO Sensitivity, SNR and DR etc.

I got quite a surprise when I noticed that DXO seemed to be claiming that the ISO sensitivity of the 50D at ISO 100 is the same as at ISO 200, and that as a consequence their graphs for SNR and DR etc did not even include the results for ISO 100.They seemed to be implying that ISO 200 was the real base ISO for the 50D, and that ISO 100 is an 'expanded' ISO, as it is in the Nikon D3 and D700. Yet there is no mention of this in the Canon handbook and no menu-setting for expanding ISO 200 to ISO 100.

Of course, my immediate reaction was not to fall off my chair, but to go out and take a few shots of high-dynamic-range scenes, at ISO 100 and 200 for the purpose of comparison.

I was very surprised to discover that I could not see any difference in any significant respect at all between shots taken at an exposure of twice the shutter speed at ISO 200, compared with half the shutter speed at ISO 100. At half the shuitter speed, at ISO 100, the sensor receives twice the number of photons. What has happened to those photons, I asked myself? Why are the highlights in the ISO 100 shots not blown when both shots at both ISO's are ETTR? If the highlights are not blown in both shots compared, why does the ISO 100 shot not have cleaner shadows? What electronic processes have Canon employed? It's as though they have created the equivalent of an electronic neutral density filter.

I'm completely mystified as to what's going on here, and a bit annoyed with myself for not discovering this situation sooner. I've got lots of photos taken in places like the Hermitage in St Petersburg, which I now realise could have been a bit sharper if I'd used either a faster shutter speed at ISO 200, or an aperture of F4 at ISO 200 instead of F2.8 at ISO 100.

An interesting and intriguing post Ray - thanks for sharing the info.I need to investigate this a bit - not so much for the principle but to get the low-down for the 5D mark III sensor to be able to really leverage its characteristics when shooting.

to get the low-down for the 5D mark III sensor to be able to really leverage its characteristics when shooting.

Hi TJ,

I don't shoot Canon, but from what I can tell the 5DIII uses post-ADC digital gain only after ISO 12800, so no point raising ISO beyond that unless you need 'properly' bright images SOOC. Looking at Bill Claff's graph, it seems that you do get a benefit in shadow SNR by raising ISO in-camera from 100 all the way up to 12800, although the law of diminishing returns needs to be considered after about ISO 2500: raising it from 2500 to 12800 only nets you a 0.2 stop improvement in shadow SNR at the expense of 2.3 stops of reduced DR: a decision best evaluated each time based on the situation (blips in the curve above 12800 are suspicious, as the data up there is often 'cooked').

The other take-away from the curve is that if you are after best SNR/DR it appears that you are better off increasing ISO one stop at a time (as opposed to in 1/3 or 1/2 stop increments) in correspondence of the steps in the graph: IOW if you are sitting at ISO 300 and you feel the urge to increase ISO a little, there is no point going to 400 or 500. Just leave it there with the benefit of slightly better DR until you feel the need to go to 600.

Jack

PS the 5DIII appears to have an effective QE of about 14% and PRNU of about 0.4%, plus a fairly noisy analog amplifier contributing about 8 ADU random noise throughout the range, so the last three bits never contain much information.

Thanks for the confirmation Jack.Perusal of Bill Claff's graphs does seem to point that way.Of course it isn't a one-way street because DR is important, to me anyway, so knowing where the optimal trade-offs are, one for the other, is where I am at.It is interesting how many of these curves see-saw around with variations from stop to stop and which ISO's become the preferred stop-off points (pardon the pun) and the fact they are approximately one full stop apart.

It is also interesting seeing how differently Canon versus Nikon (Sony) sensors behave.Obviously how one shoots in practice is just so different.For me, I have never really used any Nikon cameras, late or early models, but have assisted others with their Nikon cameras.Clearly once the fundamentals are mastered for me to be of further help would require knowing the fundamentals of those sensors since they behave so differently to my Canon cameras.

This thread is a handy reminder that there is no such thing as a 'generic' sensor anymore and we haven't even really started talking about Sigma Foveon sensors yet!Worse yet is the fact that the next generation of cameras (sensors) that we buy may make a lot (all?) of the current recommendations (sensor for sensor of course) redundant.Still, batting this information around helps to seat it soundly in one's brain.

What is happening "north" of 12800 on the 5D3? (or did you answer that as "suspicious")

Hi John,

To see at what in-camera ISO the 5DIII starts to use digital amplification I downloaded DPR's Studio Scene Comparison Raw files and looked at their histograms with RawDigger. It is clear that at ISO 12800 every value is present, at 25600 every other Raw value is missing and at 51200 three out of four are missing - so it would seem that at those ISO levels the output of the sensor is amplified analogically to ISO 12800, then fed to the ADC and the ADC's output is then multiplied by two (25600) or four (51200) before being written to the Raw data.

Everything out of the ADC gets multiplied by two or four, signal and noise alike, but the signal fed to the ADC is the same as at ISO 12800. So the SNR should be pretty well the same as well, all else (including Exposure) being equal. But per Bill's graph the SNRs don't stay the same. They get better. A little noise reduction before writing to the file? That would be a fair guess

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Also, when you speak of 2500ISO, it is really 3200 as long as you understand Canon's "game" of +/-EC, right?

Not being a Canon guy I am not sure what the game is. But it means the ISO setting that is 1/3 of a stop below what is labelled as ISO 3200 in-camera.

PS the 5DIII appears to have an effective QE of about 14% and PRNU of about 0.4%, plus a fairly noisy analog amplifier contributing about 8 ADU random noise throughout the range, so the last three bits never contain much information.

Jack, I'm thinking about trying to model this camera and compare it to the D800. Can you tell me the full well capacity and the unity gain ISO? The standard deviation of the ADU random noise would be nice -- I'll assume shot noise if you have no information. I'll assume the standard deviation of the pre-amp noise is that of shot noise, and I can calculate the pre-amp mean noise from your graph once I have the unity gain ISO.